Re: [PATCH 0/6] iommu: Enable devices to request non-strict DMA, starting with QCom SD/MMC

[Saravana Kannan]

On Tue, Jun 22, 2021 at 1:02 PM Rob Herring <robh@xxxxxxxxxx> wrote:
>
> On Tue, Jun 22, 2021 at 09:06:02AM -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Tue, Jun 22, 2021 at 4:35 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > >
> > > Hi Doug,
> > >
> > > On 2021-06-22 00:52, Douglas Anderson wrote:
> > > >
> > > > This patch attempts to put forward a proposal for enabling non-strict
> > > > DMA on a device-by-device basis. The patch series requests non-strict
> > > > DMA for the Qualcomm SDHCI controller as a first device to enable,
> > > > getting a nice bump in performance with what's believed to be a very
> > > > small drop in security / safety (see the patch for the full argument).
> > > >
> > > > As part of this patch series I am end up slightly cleaning up some of
> > > > the interactions between the PCI subsystem and the IOMMU subsystem but
> > > > I don't go all the way to fully remove all the tentacles. Specifically
> > > > this patch series only concerns itself with a single aspect: strict
> > > > vs. non-strict mode for the IOMMU. I'm hoping that this will be easier
> > > > to talk about / reason about for more subsystems compared to overall
> > > > deciding what it means for a device to be "external" or "untrusted".
> > > >
> > > > If something like this patch series ends up being landable, it will
> > > > undoubtedly need coordination between many maintainers to land. I
> > > > believe it's fully bisectable but later patches in the series
> > > > definitely depend on earlier ones. Sorry for the long CC list. :(
> > >
> > > Unfortunately, this doesn't work. In normal operation, the default
> > > domains should be established long before individual drivers are even
> > > loaded (if they are modules), let alone anywhere near probing. The fact
> > > that iommu_probe_device() sometimes gets called far too late off the
> > > back of driver probe is an unfortunate artefact of the original
> > > probe-deferral scheme, and causes other problems like potentially
> > > malformed groups - I've been forming a plan to fix that for a while now,
> > > so I for one really can't condone anything trying to rely on it.
> > > Non-deterministic behaviour based on driver probe order for multi-device
> > > groups is part of the existing problem, and your proposal seems equally
> > > vulnerable to that too.
> >
> > Doh! :( I definitely can't say I understand the iommu subsystem
> > amazingly well. It was working for me, but I could believe that I was
> > somehow violating a rule somewhere.
> >
> > I'm having a bit of a hard time understanding where the problem is
> > though. Is there any chance that you missed the part of my series
> > where I introduced a "pre_probe" step? Specifically, I see this:
> >
> > * really_probe() is called w/ a driver and a device.
> > * -> calls dev->bus->dma_configure() w/ a "struct device *"
> > * -> eventually calls iommu_probe_device() w/ the device.
> > * -> calls iommu_alloc_default_domain() w/ the device
> > * -> calls iommu_group_alloc_default_domain()
> > * -> always allocates a new domain
> >
> > ...so we always have a "struct device" when a domain is allocated if
> > that domain is going to be associated with a device.
> >
> > I will agree that iommu_probe_device() is called before the driver
> > probe, but unless I missed something it's after the device driver is
> > loaded.  ...and assuming something like patch #1 in this series looks
> > OK then iommu_probe_device() will be called after "pre_probe".
> >
> > So assuming I'm not missing something, I'm not actually relying the
> > IOMMU getting init off the back of driver probe.
> >
> >
> > > FWIW we already have a go-faster knob for people who want to tweak the
> > > security/performance compromise for specific devices, namely the sysfs
> > > interface for changing a group's domain type before binding the relevant
> > > driver(s). Is that something you could use in your application, say from
> > > an initramfs script?
> >
> > We've never had an initramfs script in Chrome OS. I don't know all the
> > history of why (I'm trying to check), but I'm nearly certain it was a
> > conscious decision. Probably it has to do with the fact that we're not
> > trying to build a generic distribution where a single boot source can
> > boot a huge variety of hardware. We generally have one kernel for a
> > class of devices. I believe avoiding the initramfs just keeps things
> > simpler.
> >
> > I think trying to revamp Chrome OS to switch to an initramfs type
> > system would be a pretty big undertaking since (as I understand it)
> > you can't just run a little command and then return to the normal boot
> > flow. Once you switch to initramfs you're committing to finding /
> > setting up the rootfs yourself and on Chrome OS I believe that means a
> > whole bunch of dm-verity work.
> >
> >
> > ...so probably the initramfs is a no-go for me, but I'm still crossing
> > my fingers that the pre_probe() might be legit if you take a second
> > look at it?
>
> Couldn't you have a driver flag that has the same effect as twiddling
> sysfs? At the being of probe, check the flag and go set the underlying
> sysfs setting in the device.

My understanding of what Robin is saying is that we'd need this info
well before the driver is even available. The pre_probe() is
effectively doing the same thing you are suggesting.

> Though you may want this to be per device, not per driver. To do that
> early, I think you'd need a DT property. I wouldn't be totally opposed
> to that and I appreciate you not starting there. :)

Which is what I'm suggest elsewhere in the thread:

https://lore.kernel.org/lkml/CAGETcx83qCZF5JN5cqXxdSFiEgfc4jYESJg-RepL2wJXJv0Eww@xxxxxxxxxxxxxx/

-Saravana